There are a number of surgical procedures, particularly those involving the suturing of delicate bodily tissues, in which it is customary to employ a pledget or cushioning pad with the suture in order to prevent the suture strand from cutting into the tissue. For instance, see FIG. 6, part No. 40, of Cooley, U.S. Pat. No. 4,164,046, for a description of the use of pledgets in heart valve surgery. Such pads must combine the properties of softness, adequate tear strength, and tissue compatibility in order to be useable in this application. Typical materials that have been employed to produce such surgical buttressing pledgets in the past are pads of polytetrafluoroethylene or polyester felt, such as is described in Mandel et al., U.S. Pat. No. 4,034,850, and in Cooley, op. cit. supra.
While the polyester or the polytetrafluoroethylene felted materials that have been employed as pledgets in the past have served their purpose well, there is room for improvement. For instance, the felted materials can occasionally exfoliate (i.e., release particles), which is considered to be undesirable. Also, in many cases tissue will grow into, and in some cases all the way through, the felt, which is a disadvantage if the pledget must be removed. In a certain number of heart valve installations, the valve must be replaced. In such cases, tissue ingrowth is a definite disadvantage because it makes pledget removal difficult and annulus preparation for the new valve significantly more traumatic. One additional disadvantage of polytetrafluoroethylene pledgets is that they are degraded by gamma radiation to an extent that other means must be used to sterilize them.
The present invention is based upon the discovery that certain types of segmented urethane polymers can be converted to excellent surgical buttressing pledgets for use with sutures. Such pledgets are non-linting, are compatible with body tissues, are soft, have acceptable tear strength, and permit only limited tissue ingrowth so that if the pledget must later be removed, such removal is less traumatic than is the case with the present felted pledgets. They can also be sterilized by gamma radiation.